The present invention is concerned with a method and apparatus for temperature compensation in differential pressure leak detection.
Many components produced by the manufacturing industry are checked for gas leaks by a method known as "pressure drop". The method entails filling the component with a gas (usually air) to the working pressure of the component. The supply is then switched off and the pressure is monitored. If the pressure falls then a leak is indicated.
The relationship between pressure fall and leakrate is derived from Boyles law according to which, for a fixed mass of gas: EQU .xi.pv=constant (1)
For a vessel of volume v filled to pressure, p bar, with a leak, .DELTA.v, measured at atmospheric pressure, (1 bar), then from Boyles law, EQU pv=(p+.DELTA.p)v+1.DELTA.v (2)
where .DELTA.p is the pressure change. Then EQU 0=.DELTA.pv+.DELTA.v (3)
The rate of pressure change dp/dt is given by ##EQU2## where dv/dt is the volumetric flowrate measured at atmosphere pressure 1(bar)
Thus, the rate of fall of pressure can be used as a measure of the volumetric leak from the component.
Unfortunately, however, temperature fluctuations have a similar effect on pressure within the vessel. Thus, for a mole of gas, the gas laws state that EQU pv=R.theta. (5)
where R is the gas constant and .theta. is the temperature. In changing conditions, ##EQU3##
If the rate of change of volume of the component is ignored, then to a first approximation ##EQU4## substituting from (5) gives ##EQU5##
For any fixed mass of gas with only very small temperature fluctuation during the test, then equation (8) to a first approximation becomes. ##EQU6##
In practice, the relationship between rate of change of pressure due to a fixed rate of change of temperature and the actual pressure in the vessel can be more nearly described by the second order equation: ##EQU7## where a and K are constants.
In a real test situation, it is therefore impossible to tell whether a measured pressure change is due to a small leak or to a small temperature fluctuation, or both.